Communication system

Abstract

In a dynamic frame length slotted ALOHA system the synchronizing signals are transmitted by a base station at different frequencies (each having a period equivalent to a multiple of the time slot period). The different frequencies denote different frame lengths and the central stations detect these frequencies and select a frame length accordingly.

We claim: 
     
       1. A communication system comprising: a central station (1) having means (5,3) for transmitting synchronizing signals each defining the beginning of a time slot and means (4) for selecting the intervals between synchronising signals from different integral multiples of a time slot and for providing to said transmitting means signals corresponding to said intervals; and a number of sub-stations (6,61) each having means (10,11,12) which is responsive to the timing of the synchronising signals and which is operative to transmit a message in a time slot selected from a number of time slots together forming a frame, the duration of said frame being determined by said synchronizing signals. 
     
     
       2. A communication system according to claim 1 in which the means responsive to the timing of the synchronising signal measures a time interval between two adjacent synchronising signals. 
     
     
       3. A communication system according to claim 2 in which the said number of time slots defines a "frame" of a length which is different from the said time interval. 
     
     
       4. A central station for a communication system which also includes a number of sub-stations, the central station comprising means for transmitting to each sub-station synchronising signals each defining the beginning of a time slot and means, responsive to the observed occupancy of time slots by signals received from the sub-stations, for selecting, in accordance with a predetermined algorithm, the intervals between the synchronising signals from different multiples of a time slot and for transmitting signals corresponding to said intervals to said transmitting means. 
     
     
       5. A sub-station for a communications system which comprises a central station according to claim 4 and at least one other sub-station, the sub-station comprising means which is responsive to the timing of synchronising signals received from the central station to determine the number of equal-length time slots to be allocated to a variable-length frame, and which is operative to transmit a message to the central station within a time slot selected from the frame. 
     
     
       6. A sub-station according to claim 5 in which the means responsive to the timing of the synchronising signal measures a time interval between two adjacent synchronising signals. 
     
     
       7. A sub-station according to claim 6 in which the said number of time slots defines a "frame" of a length which is different from the said time interval.